GENERAL CHEMICAL CHARACTERS OF PROTEINS 61 



phenylhydrazine, indicating the presence of aldehydic and ketonic 

 groups, and could be separated into three fractions by means of 

 phosphotungstic acid and lead acetate. This operation was carried 

 out as follows : the original product was precipitated by phospho- 

 tungstic acid (in the presence of sulphuric acid) ; to the substance, 

 regenerated by barium hydroxide from this precipitate, lead acetate 

 was added, and the greater part precipitated (fraction A) ; the filtrate 

 from the lead precipitate formed the second fraction (B, i.e., the 

 fraction precipitable by phosphotungstic acid, but not by lead 

 acetate); from the filtrate from the phosphotungstic precipitate a 

 third fraction was obtained (C), which was not precipitable with lead 

 acetate. These three fractions were hydrolysed by Emil Fischer's 

 method, but no very essential difference between them could be 

 detected. Fraction C yielded, however, practically no leucine, where- 

 as fractions A and B did. The other amino-acids obtained by 

 hydrolysis were alanine, valine, aspartic and glutamic acids. Glycine 

 and proline could not be isolated. It is noteworthy, furthermore, 

 that tyrosine, phenylalanine and tryptophane appeared to be absent ; 

 it seems probable that ozone had attacked the aromatic groups of 

 the protein molecule. 



Action of Nitric Acid. 



The principal product obtained by the oxidation of nitric acid is 

 oxalic acid. On carefully dissolving proteins in nitric acid and 

 afterwards adding water, yellow substances, known as xantho-pro- 

 teins, can be obtained. These have been investigated by von Furth 

 (Habilitationsschrift, Strassburg, 1899). 



SECTION XIX. THE ACTION OF HALOGENS ON PROTEINS. 



Investigations on the action of halogens on proteins date back to 

 1848, when Mulder described a "protein chlorous acid," which he 

 obtained as a precipitate when chlorine was passed into a solution of 

 egg-albumin. 



Since that date the preparation of halogen derivatives of proteins 

 has been the subject of numerous investigations ; owing, however, to 

 the complexity of the possible reactions, and the variations in the 

 composition of the products obtained under different conditions of 

 experiment, the preparation of halogen derivatives has, up to the 

 present, been but little applied to the characterisation of individual 

 proteins ; the composition of the products depends too much on the 

 details of the preparation. 



Chlorine Derivatives of Proteins. 



In more recent times the chlorine derivatives have been studied 

 by Rideal and Stewart, Hopkins and Pinkus, Blum and Vaubel, 

 Habermann and Ehrenfeld, and by Panzer. Rideal and Stewart, 

 and Hopkins both confirmed the older observation of Mulder with 

 regard to the formation of a precipitate when chlorine is passed into 

 a protein solution, and the two former investigators proposed to 

 found upon this reaction a method for the quantitative determina- 

 tion of proteins. 



Hopkins and Pinkus passed chlorine into a cold protein solution 

 till the latter was saturated, when a thick precipitate suddenly 

 formed; this was purified by solution in a I per cent, sodium hydroxide 



